1. Field of the Invention
The present invention relates to a piezoelectric transformer inverter. More specifically, it relates to a piezoelectric transformer inverter for lighting a fluorescent tube (a cold cathode fluorescent tube) used to backlight a liquid crystal display panel.
2. Description of the Related Art
It is common to use a fluorescent tube, such as a cold cathode fluorescent tube, to back light a liquid-crystal display in a portable information-processing device such as a mobile phone or a notebook-size personal computer. A high power alternating voltage must be used to light the fluorescent tube. Portable information processing devices, such as a notebook personal computer typically use both a battery and an AC adapter as power sources. In order for such power sources to energize a fluorescent-tube-lighting system, a DC/AC inverter for converting a low power direct current voltage from an input power supply into an alternating voltage capable of lighting the fluorescent tube, must be provided. Recently, a piezoelectric transformer inverter incorporating a piezoelectric transformer which is smaller than an electromagnetic transformer, has been under development for lighting a fluorescent tube. Such a piezoelectric transformer inverter preferably has the following performance capabilities:
(1) a wide input-voltage range so that it can be driven either by a battery or a battery charger; and PA1 (2) a wide luminance-dimming range so that power consumption can be reduced by dimming the brightness of the display screen of a liquid crystal display panel (which is equivalent to the luminance of a cold cathode fluorescent tube) and so that its operating life can be lengthened.
Japanese Unexamined Patent Publication No. 9-107684 discloses one such piezoelectric transformer inverter. The structure of this piezoelectric transformer inverter is shown in FIG. 1. The piezoelectric transformer inverter 1 includes a piezoelectric transformer 3 for applying a tube voltage to a fluorescent tube 2; a frequency control circuit 4 for detecting a tube current supplied to the fluorescent tube 2 from secondary electrodes of the piezoelectric transformer 3 to control the driving frequency of the piezoelectric transformer 3 in order to maintain the tube current at a specified value; a step-up circuit (drive circuit) 5 for allowing a frequency-dividing circuit 12 to divide the driving frequency generated by the frequency control circuit 4 and a generating drive voltage of the divided frequency which is applied to the primary electrodes of the piezoelectric transformer 3; a drive-voltage control circuit 6 for controlling the drive voltage applied to the piezoelectric transformer 3 in such a manner that it is equivalent to a specified voltage even if the input power supply voltage V.sub.S changes; and a dimming circuit 7 for performing a PWM-control of an average tube-current flowing in the fluorescent tube 2.
The step-up circuit 5 includes a pair of transistors 8 and 9, and a pair of coils 10 and 11 connected to operate as a push-pull circuit (class quasi-E operation). In a piezoelectric transformer inverter 1 performing such a push-pull operation, the drive voltage V.sub.D applied between the primary electrodes of the piezoelectric transformer 3 by switching the two transistors 8 and 9 on and off closely approximates a sine wave. Additionally, since the piezoelectric transformer inverter 1 performs a push-pull operation using the two coils 10 and 11 and transistors 8 and 9, respectively, the drive voltage V.sub.D applied to the piezoelectric transformer 3 can double the pulsed supply voltage V.sub.PS applied to the step-up circuit 5.
The frequency control circuit 4 controls the operation of step-up circuit 5 so as to ensure that the tube current flowing through fluorescent tube 2 is at a desired level. To this end, the tube current is applied to a current-voltage conversion circuit 13 which converts the current I.sub.1 flowing into the fluorescent tube 2 into a voltage V.sub.1 proportional to the tube current. This voltage is rectified by a rectifying circuit 14 to generate a direct-current voltage V.sub.2 which is applied to one input of comparator 15. The direct-current voltage V.sub.2 and a reference voltage V.sub.ref are compared by the comparator 15 and a direct-current voltage V.sub.3 (indicative of the current flowing I.sub.1 flowing through fluorescent tube 2) whose magnitude varies as a function of the relative size of voltages V.sub.2 and V.sub.ref, is output from the comparator 15 and is integrated by an integrating circuit 16 to generate a driving frequency control signal V.sub.4. Oscillation frequencies of triangle waves V.sub.5 and rectangular waves V.sub.6 output from respective outputs of a voltage-controlled oscillator (VCO) 17 are controlled according to the driving frequency control signal V.sub.4. This arrangement permits the frequency of the rectangular wave V.sub.6 output from the frequency control circuit 4 to vary and permits driving-frequency control to be performed so that the current I.sub.1 flowing in the fluorescent tube 2 is maintained at a desired current value.
However, in an attempt to control the tube current I.sub.1 only as a function of the driving frequency control signal V.sub.6, when the power supply voltage V.sub.S increases, the driving frequency deviates greatly from the proximity of a resonance frequency at which the piezoelectric transformer 3 is most efficient, resulting in a significant reduction of the conversion efficiency.
To avoid this problem, a drive-voltage control circuit 6 is disposed between the step-up circuit 5 and the power supply voltage V.sub.S and converts the power supply voltage V.sub.S into a pulsed supply voltage V.sub.PS whose average value is maintained constant by varying the duty cycle at which the switching device 19 of the drive-voltage control circuit 6 is switched on and off. In the drive-voltage control circuit 6 the drive voltage applied to one of the primary electrodes of the piezoelectric transformer 3 is rectified by a rectifying circuit 20 to convert it into a direct current voltage V.sub.7 which is applied to one input of a comparator 21. The comparator 21 compares triangle waves V.sub.5 output from frequency control circuit 4 with the direct current output V.sub.7 of the rectifying circuit 20 to output rectangular waves V.sub.8 whose duty cycle varies as a function of both the drive voltage V.sub.D applied to the piezoelectric transformer 3 and the tube current I.sub.1 flowing through the fluorescent tube 2. When the output of comparator 21 is low (L), the switching device 19 is turned on. When the output of comparator 21 is high (H), the switching device 19 is turned off. This forms a feedback circuit which maintains the drive voltage V.sub.D applied to the piezoelectric transformer 3 substantially constant.
The feedback circuit operates as follows. When the power supply voltage V.sub.S decreases, the drive voltage V.sub.D of the piezoelectric transformer 3 decreases as does the direct current voltage V.sub.7 output from the rectifying circuit 20. In response to the decreasing drive voltage V.sub.D, the duty ratio (the ON-duty) of the switching device 19 increases and the average value of the pulsed supply voltage V.sub.PS supplied to the step-up circuit 5 increases thereby increasing the drive voltage V.sub.D to the desired value. In contrast, when the power supply voltage V.sub.S increases, the drive voltage V.sub.D increases and the direct current voltage V.sub.7 which is output from the rectifying circuit 20 increases, therefore decreasing the duty ratio of the switching device 19 and the average value of the pulsed supply voltage V.sub.PS (and therefore the drive voltage V.sub.D) is reduced. In this way, even if the power supply voltage V.sub.S varies, the average drive voltage V.sub.D supplied to the piezoelectric transformers can be maintained substantially constant, so that the control-varying width of the driving frequency signal V.sub.5 output by frequency control circuit 4 can be set small enough to be able to adapt to a wide range of input voltages V.sub.S.
The dimming circuit 7 adjusts the dimming range of the fluorescent tube 2 as a function of a dimming voltage applied thereto. In the dimming circuit 7, the comparator 23 compares triangular waves V.sub.9 output from a triangular wave generation circuit 22 with the dimming voltage and generates rectangular waves V.sub.10 as an output. When the dimming voltage increases, the duty ratio of the rectangular waves V.sub.10 output from the comparator 23 decreases and vice-versa.
An OR gate 18 is connected to a control terminal (gate) of the switching device 19 which is disposed in the drive voltage control circuit 6. The rectangular waves output from the dimming circuit 7 have a substantially lower frequency than the rectangular waves V.sub.8 output from the comparator 21. While the duty cycle of the rectangular waves V.sub.8 generated by comparator 21 control the average value of the pulsed supply voltage V.sub.PS during the time periods in which tube 2 is turned on (and thereby stabilize the tube current I.sub.1 at a desired value), the duty cycle of the rectangular waves V.sub.10 generated by a dimming circuit 7 control the time periods in which the fluorescent tube 2 is on and thereby control the apparent rightness of the light generated by tube 2.
More particularly, the tube current I.sub.1 flows intermittently and the fluorescent tube flickers on and of with a frequency and duty cycle determined by rectangular waves V.sub.10. If the frequency of the flicker is set to approximately 210 Hz, the flicker will be imperceptible and will look as if the luminance of the fluorescent tube is being dimmed. Accordingly, by changing the duty cycle at which the switching device 19 is turned on and off it is possible to achieve a wide dimming range.
However, the above conventional circuit has the following technical problems. FIGS. 2A through 2F show the operating states of piezoelectric transformer inverter 1, when the output V.sub.10 of the dimming circuit changes from low to high and back to low. FIG. 2A shows the waveform of the rectangular pulses V.sub.10 generated by the dimming circuit 7 (the output of comparator 23). FIG. 2B shows the waveform of the triangular waves V.sub.5 generated by the frequency control circuit 4 (one output of the voltage-controlled oscillator 17) and the waveform of the DC output V.sub.7 appearing at the output of the rectifying circuit 20. FIG. 2C shows the pulsed output signal V.sub.8 generated by the comparator 21 of the drive voltage control circuit 6. FIG. 2D shows an output V.sub.11 of the OR gate 18, FIG. 2E shows the pulsed supply voltage V.sub.PS appearing at the output of the drive voltage control circuit 6, and FIG. 2F shows the drive voltage V.sub.D applied to the piezoelectric transformer 3.
As shown in FIGS. 2B and 2C, the direct current voltage V.sub.7 (which is indicative of the magnitude of the drive voltage V.sub.D applied to the piezoelectric transformer 3) is compared to the triangular wave V.sub.5 from the frequency control circuit 4 in comparator 21 which generates a pulsed output V.sub.8 as a function thereof. This signal is OR'ed with the dimming signal V.sub.10 to control the operation of switching device 19 and thereby control the generation of the pulsed supply voltage V.sub.PS as shown in FIGS. 2A, 2D, and 2E.
However, during a period in which the drive voltage control circuit 6 is intermittently turned off by the dimming signal V.sub.10 (that is, during a period in which the dimming signal V.sub.10 is high (H)), the drive voltage V.sub.D is zero, as shown in FIG. 2F. As a result, output V.sub.7 of the rectifying circuit 20 is reduced and the output V.sub.8 of the comparator 21 (FIG. 2C) is low (L). Subsequently, when the dimming signal V.sub.10 returns to its low state (L) and the drive voltage control circuit 6 restarts operation, the output V.sub.7 of the rectifying circuit 20 is increased, and the pulsed supply voltage V.sub.PS is increased to reach a desired value.
As shown in FIG. 2E, during the transitional period from the restart of operation by the drive voltage control circuit 6 to the stabilization of the output V.sub.7 of the output V.sub.7 of the rectifying circuit output 20 at the desired value, the average value of the pulsed supply voltage V.sub.PS is too large because the duty ratio of the switching device 19 is too large. This increased duty cycle causes the drive voltage V.sub.D to spike during the transition period (FIG. 2F) resulting in the following problems (1) stress to the piezoelectric transformer 3 is large and (2) an FET having a large withstand voltage needs to be used for transistors 8 and 9 of the step-up circuit 5.